05 September 2017

Irma becomes monster Category 5 hurricane as it heads for Leeward Islands

Hurricane Irma continues to strengthen over the warm tropical Atlantic ocean, and is now a rare Category 5 hurricane with 175 mph winds.  It is also closing in on the northern Leeward Islands, where it is forecast to make a direct impact early Wednesday. Beyond that, Irma also threatens the Virgin Islands, Puerto Rico, the Dominican Republic, Haiti, Cuba, the Bahamas, and the United States.  South Florida is on high alert for major hurricane conditions this weekend.

Hurricane warnings are in effect for the northern Leeward Islands, the Virgin Islands, and Puerto Rico... new warnings will be added to the west as the storm tracks just north and parallel to the Greater Antilles. Storm surge heights of 7-11 feet are possible in the warning areas, as well as heavy rain that can produce flash flooding and mudslides.

The environment will support an extremely intense hurricane for the foreseeable future, and Irma could dip in and out of the Category 5 classification (sustained winds in the eyewall exceed 157 mph) over the next few days. But regardless of the exact category rating, it will be extremely dangerous and will produce the full gamut of hurricane hazards from huge storm surges to torrential rain to severe winds capable of causing catastrophic damage.

The longer-range ensemble guidance is in strong agreement on a sharp northward turn on Sunday morning, but the precise timing and location of the turn has huge implications for Florida.

As of Tuesday morning, it is impossible to say with certainty if Irma will track up along the eastern side of the Florida peninsula, the western side, or straight up the peninsula.  For a major hurricane, the exact track of the relatively small eyewall is really important -- the largest storm surge will occur to its right and the most violent winds in the storm are confined to that annulus around the calm eye. All of Florida, and especially south Florida, should be preparing for a major hurricane landfall on Sunday.  Tropical storm force winds will arrive later on Friday at which point outdoor activities are dangerous.

Beyond the weekend, the scenarios really depend on which side of Florida it tracks. But for now, it's safe to say that the southeast U.S., including the Florida panhandle, Georgia, and the Carolinas should also brace for potential impacts such as flash flooding, storm surge, strong winds, etc.

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